There are a number of requirements for on-board diagnostics II (OBDII) of the California Air Resources Board (CARB). Once such requirement is the ability to detect misfires in the engine in order to identify a malfunction.
One method for detecting such misfires is to produce a signal wave having a frequency variable with the frequency of exhaust noises of the engine. A signal is also produced which is variable with the output speed of the engine in order to analyze the frequency characteristics of the signal wave. This approach is illustrated in the Aono et al., U.S. Pat. No. 4,083,234.
The Meier, Jr. et al., U.S. Pat. No. 4,424,709 discloses a method for detecting engine defects in the frequency domain including cylinder misfires.
Whereas normal engine combustion involves a certain level of torque nonuniformity, misfire significantly increases the nonuniformity. This nonuniformity can be readily observed in the frequency domain. Furthermore, the torque nonuniformity results in a nonuniformity in crankshaft angular speed.